Radiation-sensitive silver iodide emulsions, though infrequently employed in photography, are known in the art Silver halide emulsions which employ grains containing silver iodide as a separate and distinct phase are illustrated in: German Patent No. 505,012; Steigmann, "Green-and Brown-Developing Emulsions," Photographische Industrie, vol. 34, pp. 764, 766, and 872; U.S. Pat. Nos. 4,094,684 and 4,142,900; U.K. Patent Application No. 2,063,499A; and Research Disclosure, Vol. 18153, May 1979. The Research Disclosure reference describes silver iodide phosphate Photographic emulsions in which silver is coprecipitated with iodide and phosphate. A separate silver iodide phase in not reported.
The crystalline configurations for silver iodide are not as well publicized primarily, because silver iodide emulsions are of limited direct utility in many photographic systems. However, crystalline structures of silver iodide have been studied by crystallographers, particularly by those interested in photography. As illustrated by Byerley and Hirsch, "Dispersions of Metastable High Temperature Cubic Siliver Iodide", Journal of Photographic Science, vol. 18, 1970, pp. 53-59, it is generally recognized that silver iodide is capable of existing in three different crystal forms. The most commonly encountered form of silver iodide crystals is the hexagonal wurtzite type, designated .beta. phase silver iodide Silver iodide is also stable at room temperature in its face centered cubic crystalline form, designated .gamma. phase silver iodide A third form of crystalline silver iodide, stable only at temperatures above about 147.degree. C., is the body centered cubic form designated .alpha. phase silver iodide. The .beta. phase is the most stable form of silver iodide
It was previously believed that more nearly cubic AgI was precipitated when silver ions were in excess and more nearly hexagonal AgI resulted when iodide ions were in excess. More recent measurements, however, indicated that the presence or absence of gelatin and the rate of addition of the reactants had pronounced effects on the amounts of cubic and hexagonal AgI. Entirely hexagonal material was produced only when gelatin was present and the solutions were added slowly without an excess of either silver or iodide. No condition was found where only cubic material was observed.
Plate-like silver iodide crystals have been observed. Preparations with an excess of iodide ions, producing hexagonal crystal structures of predominantly .beta. phase silver iodide are reported by Ozaki and Hachisu, "Photophoresis and Photoagglomeration of Plate-like Silver Iodide Particles," Science of Light, vol. 19, No. 2, 1970, pp. 59-71 and by Zharkov, Dobroserdova, and Panfilova, "Crystallization of Silver Halides in Photographic Emulsions IV, Study by Electron Microscopy of Silver Iodide Emulsions," Zh. Nauch. Prikl. Fot. Kine., Mar.-Apr., 1957, 2 pp. 102-105.
The morphologies of truncated hexagonal pyramidal and hexagonal bipyramidal silver iodide grains are described by Daubendiek in "AgI Precipitations: Effects of pAg on Crystal Growth (PB) III-23", Papers from the 1978 International Congress of Photographic Science, Rochester, N.Y., pp. 140-142. The bipyramid silver iodide crystals are described as being useful precursors for silver chloroiodide, chlorobromoiodide, and bromoiodide emulsions in U.S. Pat. Nos. 4,094,684, 4,150,994, 4,184,877, 4,184,878, and 4,414,310.
U.S. Pat. No. 4,094,684 describes composite crystals obtained by deposition of a silver salt epitaxially onto silver iodide grains. Particularly described is the deposition of silver chloride onto silver iodide host grains to obtain multi-faceted silver iodide crystals having a minimum mean diameter of at lease 0.1 micron and silver chloride crystals forming epitaxial junctions with the silver iodide crystals. At least one half of the facets of the silver iodide crystals are substantially free of epitaxial silver chloride.
U.S. Pat. No. 4,150,994 describes the preparation of silver iodobromide or of silver iodochloride emulsions, utilizing an Ostwald ripening step, which produces silver halide grains of the twinned octahedral or cubic type. This patent describes silver iodide seed grains. However, it neither teaches nor suggests a process for the preparation of silver halide grains, which grains comprise at least 90 mol percent iodide.
U.S. Pat. Nos. 4,184,877 and 4,184,878 are similar to the U.S. Pat. No. 4,150,994 and describe preparation, without use of an Ostwald ripening step, of twinned silver halide crystals formed from silver iodide grains which are predominantly of the hexagonal type. U.S. Pat. No. 4,184,877 follows the procedure of U.S. Pat. No. 4,150,994 but also includes the step of chemically sensitizing the silver halide grains.
U.S. Pat. No. 4,414,310 is directed to a process for preparing tabular grain silver bromoiodide emulsions by concurrently introducing silver and bromide salts into a reaction vessel containing an emulsion comprising a dispersing medium and high iodide silver halide grains. Prior to concurrent introduction of silver and bromide salts, the mean diameter of the high iodide silver halide grains is limited to less than 0.1 micron and the concentration of iodide is limited to less than 10.sup.-2 mole per liter.
U.S. Pat. No. 4,490,458 describes multicolor photographic elements which contain superimposed emulsion layers for separately recording blue and minus blue light. The elements include at least one emulsion layer comprised of a dispersing medium and silver halide grains, wherein at least 50 percent of the total projected area of the silver halide grains is provided by thin tabular silver iodide grains having a thickness of less than 0.3 micron and an average aspect ratio of greater than 8:1. The multicolor photographic elements show advantages in the minus blue recording emulsion layers directly attributable to the thin tabular silver iodide grains.
U.S. Pat. Nos. 4,916,052 and 4,927,745 relate to a process for preparation of radiation sensitive silver iodide grains which have the morphological configuration of four hexagonal bipyramids in which their four bases are joined to form a common tetrahedron. This process comprises the steps of:
(a) forming in a colloid dispersing medium silver halide grains containing at least 90 mole Percent iodide by homogeneous nucleation at a pAg value of from about 11.0 to about 5.0 at a temperature between about 30.degree. and 90.degree. C.; PA1 (b) maintaining these conditions until the silver iodide grains are from about 0.005 to about 0.5 .mu.m in diameter; PA1 (c) altering the growth environment of the silver iodide grains to a pAg value of from about 13.5 to about 9.8 at a temperature from about 30.degree. to about 90.degree. C., and, optionally; PA1 (d) removing water soluble salts formed during the precipitation process
Although this procedure effectively produces silver iodide grains of the desired configuration, it is desirable to better control the production of either elongated or truncated grains. In addition, the ratio of grains having the desired tetrahedral configuration of 4 hexagonal bipyramids to grains with a single hexagonal bipyramid morphology or with other undesired forms is too low for commercial utility. There is thus a need to improve this process.